Using a metal to form a connection to silicon can cause manufacturing, quality or reliability problems. Thick layers of metal are needed to achieve the low impedance necessary for desired circuit performance. However, thick layers of many metals are difficult to etch for forming a desired interconnect pattern. For this reason, aluminum is a highly desirable metal to use in semiconductor processing because it is easy to etch into desired patterns. Unfortunately, aluminum and silicon are particularly attractive to one another so that the silicon atoms will diffuse into aluminum, leaving voids in the silicon substrate. Aluminum atoms will next fill these voids, forming spikes of aluminum which destroys the aluminum to silicon junction, i.e. silicon is soluble in aluminum and has a solubility of approximately 0.5 atomic percentage at 450.degree. C.
A variety of techniques have been used to overcome this junction spiking problem. One technique to avoid junction spiking includes applying a barrier metal between the substrate and the aluminum layer that is not reactive to or soluble in either silicon and aluminum. In other words, a thin layer of the barrier metal is deposited and then covered with a thicker layer of aluminum. The thin layer of the barrier metal prevents the aluminum from spiking into the underlying silicon. The thick aluminum provides a sufficiently low impedance current carrying path to achieve desired performance criteria. While barrier metals are more difficult to etch than aluminum, they are typically thin by comparison so that etching problems are minimized. Titanium nitride (TiN) is a commonly used barrier metal.
As processing technology has advanced to provide thinner lines for greater circuit density, the aspect ratio of contact vias (the ratio of via depth to the diameter of the opening) has correspondingly increased. Because of the small geometries and the aspect ratio of the contact holes, it becomes increasingly difficult to simply fill contact vias by conventional physical deposition techniques such as vacuum evaporation and sputtering. As a result voids are formed in the holes. These voids can either form an open circuit, increase the impedance of the contact or cause a reliability problem such as low electromigration lifetime due to metal migration through a narrow current path.
To solve this problem, engineers have heated the wafer, such as with a laser, to melt the aluminum and allow it to flow to completely fill the contact via. Although this technique has been demonstrated successfully in filling a submicron, high aspect ratio contact/via, damage to the junction integraty during laser processing has been observed by the inventors. The degradation in the device junction is caused by a breakdown in the barrier layer which allows rapid silicon diffusion into the Aluminium alloy, and a resultant aluminum spike formation at laser processing temperatures as high as 1000.degree. C.-1700.degree. C. Therefore, the standard barrier schemes, which involve a single layer of barrier metal such as titanium nitride and titanium tungsten of thickness .about.1K .ANG., are not adequate in preventing junction spiking in a laser planarization process.
A semiconductor process is needed which completely fills contact vias in high aspect ratio small geometry devices and does not allow the formation of metal to semiconductor junction spikes or damage an intermediate barrier metal layer at processing temperatures as high as 1000.degree. C. or above.